Mipmapping for smoother textures in Stage3D

Table of contents

Created

2 April 2012

Requirements

Prerequisite knowledge

You should be familiar with setting up and running an ActionScript project based on the Stage3D API. You should also know about 3D perspective projection and 3D cameras. Be sure to read first the previous tutorials in this series on Stage3D:

User level

In this article I am going to introduce you to a big problem that often shows up when rendering texture mapped geometries, called "texture sampling aliasing," that can cause artifacts in the rendered scene. I will cover the available techniques for reducing the effects of this problem: bilinear filtering, mipmapping, and trilinear filtering. The accompanying sample application will also show you how to create a Stage3D-based application that uses these techniques.

The problem of aliasing with "distant" triangles

What is it with distant triangles?

In the previous articles of this series you saw how to render a textured triangle. Texture mapping simply means to take a texture image and to wrap it around a 3D object that gets rendered. You also saw how to render triangles in perspective and how to roam around your 3D scene using a 3D camera. This means that, as your camera moves, a triangle may become either big on the screen, when the camera is close to it, or very small on the screen, when the camera is far away from it.

This is all cool and dandy, and you get your neatly rendered geometry. But it is not always as simple as it seems, as there may be a few issues to deal with.

Let me first try and explain the problem intuitively. The trouble comes when you take a texture that is some size that is, say, "proper" for the screen—not too big, not too small—and you apply it to some object that is rendered as very small on the screen (for example, a faraway object in the 3D scene).

Let me put down some numbers, so that it gets clearer. Say you have a texture that is 512×512 pixels and you apply this texture to a 3D triangle. Now, if this rendered triangle ends up projected on the screen as something that's more or less on the same scale as the texture—something around 512×512 pixels on the screen—then it will all be okay, as the scaling of the texture is not going to be too severe.

On the other hand, if this triangle is far away from the eye point, so it gets rendered as a very small object on the screen—say, it fits within a 20×20 pixel area on the screen—then the texture gets strongly scaled, and this can produce some very bad artifacts on the final rendered object.

The problem is one of putting together the texture mapping with the process of scaling. Figure 1 shows a rendered texture with the actual pixel screens overlaid. You can see how several texels (pixels of the texture), fall inside a single screen pixel.

Figure 1. A checkerboard textured object viewed through a row of screen pixels.

This creates the problem of having to come up with a rendered pixel color for each screen pixel that somehow represents all the texels falling within that pixel. This whole process of picking a texel and assigning it to a pixel is called texture sampling, and it's what the Texture Sampler registers in the Fragment Shaders are made for.

The most straightforward way to do the texture sampling is to simply choose the "nearest neighbor" texel: the one that, in the texture mapping, falls closest to the screen pixel center. This method is simple, but it's also the one that creates the worst artifacts. See Figure 2 for an example of artifacts caused by nearest neighbor texture sampling.

Figure 2. Artifacts from nearest-neighbour texture sampling.

Mind you, texture sampling artifacts are not always this bad when using nearest-neighbor texture sampling. In general, they depend on the kind of texture used and on the distance and orientation of the object from the 3D camera. This is a particularly bad case. Checkered textures are often the worst in terms of texture sampling artifacts.

This problem gets even worse when the object moves around on the screen. In fact, when the on-screen position of the object changes, the sampling changes as well, so you often see a strong flickering of the artifacts that makes them extremely evident.

So, what do you do?

What is mipmapping?

A simple way to reduce the effect of texture sampling artifacts is to use a technique called bilinear filtering. Instead of just using the nearest neighbor texel, a bilinear filter takes into account four nearby texels, and blends them through linear interpolation.

This improves the situation just a little bit, as you can see in Figure 3.

Figure 3. Texture sampling using bilinear filtering

However, the most popular way to solve this problem is to use a technique called mipmapping. The "mip" stands for "multum in parvo". This comes from Latin, and it means "multiple things in a small place." In fact, mipmapping is a process where the original texture gets filtered down repeatedly into a series of smaller textures, thus creating a multitude of textures that get associated to the same pixel.

How does it work?

Instead of using just the standard 512×512 texture, you also precalculate reduced versions of the same texture: 256×256, 128×128, and so on, down to the 1×1 version. Thus, you create the so-called texture pyramid, as shown in Figure 4.

Figure 4. Mipmapping texture pyramid.

So, for a 512×512 texture, this means a total of nine textures get generated. Each of the different textures is called a mip level, and it's often indicated with the letter d (where d goes from 0 to 8 in this case).

Now, this might seem a horrible waste of GPU memory: to use nine actual texture images for every single texture of every object. But think of this: the mip level textures are smaller than the main full-size texture. And if you calculate the total space occupied by the full mipmapped texture (all the nine levels in this example), you'll see that it is only 33% times bigger than the single full size texture:

512×512 + 256×256 + 128×128 + ….1×1 < 512×512 * (1 + 1/3)

So, it's not that much of a waste of GPU memory as if you were using nine times the memory. You're just using 1.33 times the memory.

What do you do with these mip levels?

When mip levels are available for a texture, and mipmapping is enabled, then, for every triangle pixel that gets rendered, the rendering pipeline looks at how large (or small) this pixel is when mapped onto the texture and compared to the size of the texels, as shown in Figure 5. If the size of the projected pixel and the texels are about the same, then the main texture can be used without too many problems. On the other hand, if the triangle pixel is much bigger than the texels—or, in other words, if many of the texels fall inside the triangle pixel—then it means that a mip level corresponding to a reduced version of the texture needs to be used.

Figure 5. Projection of pixel cell onto texture space.

In reality, what happens is that a value for d gets calculated starting from some sort of ratio between the triangle pixel size (projected onto the texture space) and the texel size. And this value of d corresponds to the mip level that is more suitable for being used.

In the simplest case, this calculated value of d gets rounded to the nearest integer, and the pixel from that mip level texture gets used for that specific triangle pixel. This is called "nearest neighbor" mipmapping.

A more accurate way to do it is to not round off d to the nearest mip level, but to also take into account the "distance" from the two levels between which d sits, and then perform a linear interpolation of the two mip level textures that bound d, using d as a distance. This process of interpolation is called linearly filtered mipmapping. It is more accurate and produces better results, but it's also a bit heavier on the GPU.

An even more advanced way to use mipmapping is to combine linearly filtered mipmapping with bilinear filtering and obtain even better results. This is called trilinear filtering.

How to use mipmapping in Stage3D

How do you put this theory into practice, and use mipmapping with the Stage3D API? Stage3D supports mipmapping by means of the Texture class. In fact, whenever you upload a texture image to the GPU using the Texture class, you can specify, as a parameter, the mip level to which the image corresponds.

The Fragment Shader that uses the texture will then need to specify that it wants to use mipmapping when sampling the texture. This is done by using either the mipnearest or the miplinear flags when using the Texture Sampler. As the name implies, mipnearest uses nearest-neighbor mipmapping, while miplinear uses linearly filtered mipmapping.

If, on the other hand, you don't want mipmapping and just want to go for simple bilinear filtering, just use mipnone (or nomip ; they're the same) in place of mipnearest / miplinear and keep the linear flag on:

A sample mipmapping application

I'll now show you how to create an application that uses mipmapping.

Start from the code that was created in the previous article on 3D cameras, as it's useful to be able to roam around the scene and see the effects of mipmapping. But this time, you're going to be using a checkered texture instead of a rocky texture, so that the artifacts are more visible.

Instead of a rectangle spinning in the distance, you'll use a static horizontal floor plane, above which you can roam around:

This is it! The application is done. Figure 6 shows the result, with trilinear filtering enabled. Have fun roaming around the checkered floor using your keyboard arrows. I encourage you to change the flags in the Fragment Shader and test the various texture filtering settings discussed earlier:

Nearest-neighbor texture sampling without mipmapping

Bilinear filtering without mipmapping

Nearest-neighbor mipmapping with and without bilinear filtering

Linearly interpolated mipmapping without bilinear filtering.

Notice the visual differences in the result.

Figure 6. Application in action with Trilinear Filtering enabled.

To see the full source code of the application, download the sample file.

Where to go from here

This article introduced you to the problem of artifacts caused by texture sampling. It covered techniques such as bilinear filtering, mipmapping, and trilinear filtering that can be used with the Stage3D API to reduce the extent of texture sampling artifacts. This theory was then developed into the creation of a basic sample application that uses mipmapping and bilinear filtering to demonstrate the different visual results that can be obtained with these techniques.

The world of algorithms and techniques aimed at improving the visual quality of 3D rendering is extremely vast. For those who want to dive deeper, I'll recommend a book called Real-Time Rendering, by Thomas Akenine Moller, Eric Haines and Naty Hoffman.